Electrochemical chlorination of hydrocarbons in an hci-acetic acid solution

ABSTRACT

A PROCESS OF ELECTROCHEMICALLY HALOGENATING HYDROCARBONS IN AN AQUEOUS HYDROHALIC ACID ELECTROLYTE COMPRISING PASSING CURRENT FROM A CATHODE TO AN ANODE IMMERSED IN AN ELECTROLYTE COMPRISING A MIXTURE OF AQUEOUS HYDROHALIC ACID AND 10-90 VOLUME PERCENT OF AN ALIPHATIC ACID OF 2-4 CARBON ATOMS. TYPICALLY, THE PROCESS INVOLVES CHLORINATING AN ALIPHATIC, CYCLOALIPHATIC OR AROMATIC HYDROCARBON IN AN ELECTROLYTIC CELL EMPLOYING GRAPHITE ELECTRODES USING A MIXTURE OF HYDROCHLORIC ACID AND ACETIC ACID. USING THE MIXED ACID ELECTROLYTE PREVENTS RAPID DETERIORATION OF THE ANODES AND INCREASES THE SOLUBILITY OF THE HYDROCARBON STARTING MATERIAL IN THE ELECTROLYTE.

United States Patent Office Patented Sept. 19, 1972 3,692,646ELECTROCHEMICAL CHLORINATION F HYDRO- CARBONS IN AN HCl-ACETIC ACIDSOLUTION William B. Mather, Jr., Hopewell Junction, and Edwin R.

Kerr, Wappingers Falls, N.Y., assignors to Texaco Inc., New York, NY. N0Drawing. Filed Sept. 2, 1971, Ser. No. 177,484 Int. Cl. C07b 9/00, 27/06US. Cl. 20481 9 Claims ABSTRACT OF THE DISCLOSURE A process ofelectrochemically halogenating hydrocarbons in an aqueous hydrohalicacid electrolyte comprising passing current firom a cathode to an anodeimmersed in an electrolyte comprising a mixture of aqueous hydrohalicacid and 10-90 volume percent of an aliphatic acid of 2-4 carbon atoms.Typically, the process involves chlorinating an aliphatic,cycloaliphatic or aromatic hydrocarbon in an electrolytic cell employinggraphite electrodes using a mixture of hydrochloric acid and aceticacid. Using the mixed acid electrolyte prevents rapid deterioration ofthe anodes and increases the solubility of the hydrocarbon startingmaterial in the electrolyte.

BACKGROUND OF INVENTION This invention relates to a process forhalogenating hydrocarbons. More particularly, it relates to an improvedmethod for electrochemically chlorinating or brominating hydrocarbonsusing a modified solvent-electrolyte mixture which increases thesolubility of the hydrocarbon reactant in the electrolyte and aids inreducing the rate of deterioration of the electrodes duringelectrolysis.

Though for the purpose of brevity, the description of the presentinvention will be, for the most part, specific to chlorinationreactions, bromination reactions are substan' tially analogous and theyare intended to be included herein.

Electrochemical halogenation, particularly chlorination of organiccompounds is well known and widely practiced. The synthesis ofchlorohydrin from ethylene and a neutral aqueous chloride solution is acommercial process. The electrochlorination of n-dodecane with aqueousHCl has also been disclosed (Ruehlen et al.--.l. Electrochem. Soc. 111,1107 (1964)).

Electrolytic chlorinations are carried out in conventional electrolysiscells. The cell consists of a covered vessel containing electrolyte andequipped with an anode and a cathode extending down through the coverinto the electrolyte. The cell is divided into interconnected anode andcathode compartments, each compartment being provided with a port topermit escape of gases produced during electrolysis. A source of DCpower is connected across the electrodes.

Electrochemical chlorination involves the production of atomic chlorineor the chloronium ion (01+) by electrolytic dissociation of achlorine-containing compound in the electrolyte. As noted above, thechlorine compound may be one which gives neutral or acidic aqueoussolutions. To commence electrochlorination, the compound to bechlorinated is added to the anode compartment and a DC potential isapplied across the electrodes. At the cathode, hydrogen gas is formedfrom the reduction of hydrogen ions, and at the anode, chlorine isgenerated by dissociation of the chlorine-containing electrolyte. Thechlorine generated at the anode reacts with the hydrocarbon present inthe anode compartment to give the desired chlorohydrocarbon. Excesschlorine or hydrogen produced during electrolysis is vented through theelectrolysis cell ports provided for this purpose.

[In order for electrochemical chlorinations to be conducted in aneconomically feasible manner, the product yield and purity must berelatively high and the electrolytic cell, especially the electrodes,must be resistant to rapid deterioration during use.

From a standpoint of cell durability, it is preferred to conduct thechlorination in a neutral aqueous chloride solution since suchelectrolytes do not cause rapid deterioration of the cell and theelectrodes. However, the neutral electrolyte process has severaldrawbacks which have an adverse impact upon its overall efiiciency. Themain drawbacks are the lack of solubility of the hydrocarbon startingmaterial in the electrolyte, the relatively poor conductivity of theelectrolyte and the problem of by-product alkali which is expensive toseparate from residual sodium chloride. Other difiiculties, such as theinstability of the chloronium ion, necessary for the chlorination ofolefin or aromatic hydrocarbons is much less stable in neutral than inacid solution.

The disadvantages attendant the use of a neutral electrolyte forchlorination of hydrocarbons are substantially avoided when thechlorination is carried out in a simple aqueous HCl electrolyte. Thoughit is advantageous from one standpoint to use an aqueous acidicelectrolyte, aqueous acids cause rapid deterioration of the anodeespecially when a graphite anode is employed. Moreover, hydrocarbonsused as starting materials are not generally soluble in aqueous HCl andsince the solubility of the starting material is a determinant inproduct yields, yields of product based upon current input have not beenas high as desired.

'In view of this state of the art, it is an object of the presentinvention to provide a modified electrochemical chlorination process forconverting hydrocarbons to chlorinated derivatives thereof. It isanother object of the present invention to provide a new method ofelectrochemically chlorinating hydrocarbons wherein the elec trolysiscell, especially the electrodes thereof, is not subject to rapiddeterioration during electrolysis. Other objects will be apparent fromthe ensuing description of this invention.

SUMMARY AND DESCRIPTION OF INVENTION In accordance with one aspect ofthis invention, it has been found that electrochemical halogenation,e.g., chlorination and bromination, can be conducted efliciently in amanner which does not result in rapid deterioration of the electrodes.The method involves conducting the electrolysis in an electrolytecomprising a carboxylic acid of 2-4 carbons such as glacial acetic acid,propionic and/or butyric acids and aqueous concentrated hydrochloric orhydrobromic acid. It has been found that the carboxylic acid providestwo beneficial effects. It increases the solubility of the hydrocarbonsin the electrolyte and it reduces the deterioration of the anode. Dryhydrogen halides can be used, but higher conductivity and better currentefficiencies are achieved with aqueous concentrated hydrohalic acids,e.g., 12 N HCl or HBr. A preferred electrolyte contains 50% by volume ofglacial acetic acid and 50% by volume of 12 N HCl. However, beneficialeffects are obtained when as little as 10% by volume of carboxylic acidor as much as is in the electrolyte. When the concentration ofcarboxylic acid is less than about 10%, solubility of the hydrocarbonsis very low and deterioration of the graphite anode occurs. Too high aconcentration of glacial acetic acid reduces the conductivity of theelectrolyte thereby considerably increasing the lvoltage requirementsfor the reaction and raising the operating costs for the process.

The process of this invention is applicable to aliphatic, cycloaliphaticand aromatic hydrocarbons in general so long as they are soluble in theelectrolyte. Examples of useful compounds are parafiins of 3 to 20carbon atoms such as propane, nonane, cetane, etc.; cycloalkanes of 4 to12 carbon atoms such as cyclobutane, cyclohexane, etc.; olefins of 2 to20 carbons such as ethylene, dodecene- 1, etc.; cycloalkenes of 4 to 12carbons such as cyclobutene, cyclohexene, etc.; and aromatics of 1-3carbocyclic rings such as benzene, naphthalene, biphenyl anthracene andlower C alkyl or alkenyl substitution products thereof such as toluene,xylene, cumene and styrene.

The hydrocarbon reactant need not be completely soluble in theelectrolyte for the chlorination to proceed. However, chlorination bythe process-f this invention is more efiicient for olefinic and aromaticcompounds, than for the less soluble paraffins. Gaseous, liquid andsolid hydrocarbons can be chlorinated so long as the reactanthydrocarbon is brought into intimate contact with the electrolyte. Inthe case of gaseous hydrocarbon reactants, the hydrocarbon is bubbledinto the electrolyte by positioning the reactant feed inlet below thelevel of the liquid electrolyte in the anode compartment. Solidreactants can be comminuted prior to being introduced 'or dispersed byhigh shear agitation in the electrolyte cell, and maintained insuspension by agitation during electrolysis. Liquid hydrocarbonreactants which do not dissolve in the electrolytic cell can besuspended in the electrolyte by agitation during the chlorinationreaction.

The chlorination process of this invention advantageously can beconducted in conventional electrolysis cells of the aforedescribed type.The electrodes can be fabricated of graphite, preferably of the highdensity type. One advantage of using a carboxylic acetic acid solvent inthe electrolyte is that it permits use of low cost graphite electrodeswithout the previously encountered serious problem of rapid electrodedeterioration due to spalling away of carbon during electrolysis. Notonly does this s-ur prising result beneficially influence the durabilityand efiiciency of the cell, but it also leads to the production ofchlorinated hydrocarbon product not contaminated by particles of thegraphite electrodes.

The yield of product is determined by amount of current used, time ofelectrolysis, time to quench, voltage, temperature, electrolytecompaction, and in certain cases, the presence or absence of short waveUV radiation.

The amount of current theoretically required is that which yields thenumber of electrons required to-eifect a particular chlorinationreaction. One equivalent of electrons is required for monochlorination,two for dichlorination, etc. The actual current requirement is basedupon the percent current efiiciency, i.e., the number of'electrons usedto effect a particular reaction divided by the total electrons used andmultiplied by 100. Current efficiency for any particular reactionincreases until enough chlorine has been generated to react with thehydrocarbon starting material. The increase in current elliciencyappears to be due to an initiation period in which current does not formproducts, because, it is speculated a certain concentration ofchlorinating species (about 0.03 equiv.) must be generated beforechlorination occurs. After the initiation period, there is a rapidincrease in product formation until nearly all the hydrocarbon hasreacted. After the equivalence point, current efficiency declines sincethere is no more hydrocarbon to react. Thus, the optimal use of currentis fortunately achieved at highest starting material conversion levels.The electrolysis can be conducted at any temperature between ambienttemperatures and the boiling point of the electrolyte.

In most electrochlorinations in accordance with this invention, currentefiiciency and product selectivity are not greatly influenced byincident light and generally the reaction proceeds equally well in roomlight and with no light. However, the bromination of parafiins issignificantly affected by radiation which is capable of dissociatinghalogen molecules into halogen atoms. Wavelengths shorter than 500 mpwill dissociate halogen molecules into halogen atoms to some extentalthough maximum dissociation is around 340 III/1. or lower. In the caseof parafiin halogenation the active species is atomic halogen andtherefore the higher level of halogen dissociation aids in theattainment of greater conversion rates for the-paraffin stratingmaterial. In the case of olefins or aromatic hydrocarbons, the activespecies is either the chloronium or the bromonium ion. Since UV lightdoes not facilitate the formation of the latter, and in fact, mayinhibit their formation, only a minor or even a negative effect isnoticed when aromatics or olefins are halogenated in the presence oflight with wavelengths shorter than 500 my.

The major products of the electrohalogenation reaction of the presentinvention and mono and dihalogenated derivati'ves of the hydrocarbonstarting material. In the case of olefin starting materials, the majorproducts are dihalo derivatives of the starting materials, whereas inthe cases of paraffin and aromatic compounds the major products aremonohalo derivatives of the starting materials. The productsofhalogenation can be separated from the electrolyte by conventionalextraction or chromatographic separation techniques, the choice of whichis dependent upon the nature of the product and the degree of productpurity which is desired.

The following examples are presented to further ill-ustrate the presentinvention.

EXAMPLE 1 Chlorination of dodecene-l An electrolysis cell was made froma laboratory beaker which was fitted with a cover through which waspassed a /3" graphite rod anode and a 1" (I.D.) bottomless Pyrex tubeextending to A above the bottom of the beaker. Another graphite rod wasused as the cathode within the Pyrex tube. The beaker was equipped witha magnetic stirrer and a Teflon-coated'stirring bar.

When the electrolysis was conducted in the absence of light, the beakerwas wrapped with black plastic tape to exclude all outside light. Astrip of the tape was removable to give an area 2 centimeters wide and 5centimeters high to admit light into the cell. 8 watt long wave (about360 m peak) and short wave (about 254 m peak) dis charge bulbs wereplaced next to the untaped area of the electrolysis beaker. The Pyrexglass in the beaker transmits ultraviolet light down to 280 mp The shortwave length UV light had a peak intensity at 254 mp but a long radiationtail into the visible region. The long wave UV light had a peakintensity at 360 m and a short wave cutoff at 300 mp. Since chlorineabsorbs radiation below 480 m to form chlorine atoms, both lightsdissociated electrogenerated chlorine molecules.

The electrolyte, usually consisting of glacial acetic acid andconcentrated aqueous HCl, was mixed with the hydrocarbon in the cell andthe solution (or suspension) was stirred vigorously. A current waspassed through the cell and voltage and current measurements were taken.After a predetermined time, the current was shut off and the reactionwas allowed to stand for some hours in order to allow the chlorine toreact further. The mixture was then diluted with an equal volume ofwater and the reaction quenched with excess sodium sulfite. Productswere separated directly or extracted with chloroform and then weighedand analyzed.

Analyses were carried out on an Aerograph model 202 gas chromatograph(T.C. detector) using a 5 ft. by A" column of 20% SE-30 on 60/80Chromsorb W with isothermal operation. Effiuent peaks were collected andidentified by IR or NMR quantitative determinations.

Using the foregoing equipment and procedures, a series of experimentswas conducted in which dodecene-l was electrochlorinated in 250milliliters of an electrolyte consisting of 50% glacial acetic acid and50% 12 N HCl. Current, concentration, time of electrolysis, time toquench, equivalents of current and light conditions were comparison, tworuns were made in which an equivalent amount of sodium chloride wassubstituted for HCl as the source of chloride ions in the electrolyte.The results are presented in Table H.

TABLE IL-ELECTROCHLORINATION OF N-NONANE, N-DODECANE Gas chromatographicanalysis (moles in product) Current Starting Current Products efi. formaterial used separated UV Chloro- Dichloro- Triehlorochlorination,n-Nonane (moles) (equiv.) (moles) light Nonane nonane nonane nonanepercent 056 061 042 sw...- 038 003 9. 056 238 040 sw.--- .025 .012 7.6140 476 124 SW. 072 044 12. 140 1. 245 144 sw.-.- 011 062 17. 3 140 539127 sw--.. 120 006 1. 6 140 289 135 sw- 089 042 17. 4 140 2. 00 078sw.... 018 057 3. 2 140 .887 138 No 125 .011 1. 6

Chloro- Dichloro- Dodecane dodecane dodecane 110 1. 71 088 No- 079 0. 66110 713 125 sw..-. 073 10. 2 110 1. 033 097 sw---. .060 3. 6 110 1. 29076 sw.-.- 044 2. 6 110 .478 112 sw-... 079 8. 6 110 597 123 sw.... .06412. 6 110 1. 47 093 sw.-.. 023 7. 3 110 3.14 107 sw... 014 4. 4 .1101.255 .082 No .081 .09

experiment were isolated,

varied. Products from EXAMPLE 3 Using the equipment and following theprocedures described in Example 1, benzene and cumene were electro- Gaschromatographic analysis (moles in product) Current Starting CurrentProducts 1,2-dieh1o 2-acetoxyefi. for material used separated U.V.Dodec- Chlororododel-chlorochlorination, (moles) (equiv.) (moles) lightene-l dodecane cane dodecane percent 225 1. 00 163 041 41 225 46 166 01980 113 166 049 007 68 115 074 016 001 60 115 238 075 017 77 113 047 011002 55 113 392 074 023 113 609 083 019 33 113 331 074 020 57 113 082 0150003 46 113 117 024 004 48 113 037 0021 003 17 113 179 039 009 113 174031 009 43 113 065 009 002 34 113 024 0027 2s 113 056 011 003 55 113 284072 023 68 113 119 027 007 113 406 083 017 50 113 068 009 002 35 0460306 0029 0004 24 047 0164 044 1w- 0422 0004 0013 0001 19 046 0080 0431w- 0417 0003 0008 0002 29 .045 .0312 .044 sw..-- .0411 0008 0022 .000319 047 0134 046 sw- 0438 0009 0012 0001 26 045 0609 0445 sw. 0396 00140029 0005 13 N o attempt to exclude light.

EXAMPLE 2 55 Using the procedures and equipment described in Example 1,n-nonane and n-dodecane were electrochlorinated under varying conditionsof current, light, time, etc. and the products were isolated andanalyzed. For purposes of chlorinated in 250 milliliters of a 50%glacial acetic acid and 50% 12 N HCl electrolyte. The results arepresented in Table III.

TABLE IIL-ELECTROCHLORINATION OF BENZENE AND CUMENE Gas chromatographicanalysis Current (moles of product) efi. for Starting Current Productschlorinmaterial used separated Chloroo-Dichlop-Dichloation, Benzene(moles) (eqniv.) (moles) Benzene benzene robenzene robenzene percentCumene o-Chlorop-Chlorocumene cumene 1 All runs except No. 7 were run intaped beaker (no exposure to light) ultraviolet light (360 mp).

. Run No. 7 was exposed to long wave From the foregoing examples, it canbe seen that proding monochlorocyclohexane and dichlorocyclohexane wasuct yields and product selectivity are quite high in the obtained. caseof olefins and aromatics, and considerably lower in In the cases ofExamples and 6, there were no signs of the case of paraffins. Thus,dodecene-l was chlorinated to deterioration of the graphite anode afterelectrolysis had a mixture containing 1,2-dichlorododecane (70% selec- 5been completed. tivity) and Z-acetoxy-l-chloro-dodecane (20%selectivity) What is claimed is: representing a 50% yield of thedichloro compound. 1. In the process of electrolytically halogenatinghydro- Paratfins were chlorinated in the same system with about carbonsin an aqueous hydrochloric or hydrobromic acid 1% current efficiency inthe absence of light, but up to electrolyte by passing current from acathode to an anode 17% current efficiency in the presence of shortwaveultra- 1O immersed in said electrolyte whereby active halogen violet(254 mp. peak) light. Aromatics, like olefins, were species is formedand reacts with the hydrocarbon to prochlorinated with high currentefliciency. The initial prodduce the desired chlorinated or brominatedderivatives, uct of benzene chlorination was chlorobenzene formed in theimprovement which comprises incorporating in the about 40% yield.Further chlorination yielded only o-dielectrolyte between and 90% byvolume of an aliphatic chlorobenzene (33%) and p-dichlorobenzene (66%)with carboxylic acid of 2-4 carbon atoms. no more than 5% of the metaisomer. Overall current 2- Th pr ss of claim 1 wherein the anode andcathefliciency for benzene chlorination was therefore between Ode arefabricated 0f graphite and 50%. Cumene was chlorinated in a 50% yield toThe process of claim 1 wherein the hydrocarbon i o-chlorocumene (49%)and p-chlorocumene (51%) with n l fi f 220 r n msno more thann-chlorocumene in th product, 20 4. The process of claim 1 wherein thehydrocarbon is Analogous bromination products are obtained when the anaromatic compound of cafbocyclic ringsforegoing procedures are modifiedby substituting HBr for The Process of Claim 1 wherein the hydrocarbonis HCl in the electrolytic composition. a parafiin of 3-20 carbon atoms.

EXAMPLE 4 v 6. The process of claim 5 wherein the electrolyte is ex- 5posed to short wave UV radiation during the halogena- Using theequipment and the procedures of Example 1, tion. toluene was chlorinatedin two separate runs with and 7. The process of claim 1 wherein thecarboxylic acid without glacial acetic acid in the electrolyte. Thereaction is glacial acetic acid.

conditions and results are presented in Table IV.

TABLE IV.ELECTROCHLORINATION OF TOLUENE Starting material Current usedRun Solvent (mL) plus Voltage Time Product No. Gm. Moles Anodeelectrolyte (gm.) applied Current Amph Equiv. of elec. sep. (gm) Remarks1 43.4 .470 graphite rod.-- HOAc (125) plus H01 (125)-..- 4. 3-4.7.26.30 34.8 1.30 119 43. 1 Rirl lgwstlllqgstitution 2 43.4 .470 .do H20(125) plus H01 (125) 2.6-5.5 .17-.30 14.9 .56 72 Anode badly attacked.

From the results of Example 4, it can be noted that the 8. The processof claim 1 wherein hydrochloric acid is presence of glacial acetic acidin the electrolyte was critiemployed. cal to the attainment of asubstantial amount of the prod- 9. The composition of claim 1 whereinthe electrolyte net and the preservation of the graphite anode. is a%-50% by volume mixture of aqueous concen- EXAMPLE 5 trated hydrohallcacid and glacial acetic acid.

Electrochlorination of methylcyclopentane 4,5 References Cited Followingthe procedure of Example 1 except for the UNITED STATES PATENTSsubstitution of methylcyclopentane for dodecene-l, a 3,632,489 197Weinberg at a]. 4 product having the expected monochlorocyclopentane anddichlorocyclopentane was obtained. 0 JOHN H. MACK, Primary ExaminerEXAMPLE 6 k R. L. ANDREWS, Assistant Examiner Electrochlorlnatlon ofcyclohexane USI CL Following the procedure of Example 1 except for the204-1571 R substitution of cyclohexane for dodecene-l, a product hav-

